Hi
We had ours done a few years ago and there are a few lessons that we've learnt
The dust is incredible. It's not actually possible to create a picture of how bad the dust it, imagine a child dumping bags and bags of dust all over ALL of your house every day. It's worse than that
We lived in whilst ours was done but it was a nightmare.
Cost wise they start at about 40k and go to about 100k depending on what sort of work you're doing. They are relatively simple builds but you can find drains and electrics and planning all start to make something complicated quite awkward.
Side returns work best when you "open up" the whole of the back and although that takes quite a bit of steel its an amazing result, I honestly smile every day I do into mine.
If you have any other questions please shout